Decide and document the runtime boundary for libtortillas

[artrixdotdev]

Problem

libtortillas is tightly coupled to Tokio, Kameo, Reqwest, uTP, and async runtime primitives. That can be acceptable, but the frontend contract should explicitly say whether this is a Tokio-only core or whether runtime/network/storage injection is a goal.

Relevant code/dependencies:

• crates/libtortillas/Cargo.toml
• crates/libtortillas/src/engine/mod.rs examples use #[tokio::main]
• Direct reqwest::get calls in engine/tracker code

Acceptance criteria

• The crate docs explicitly state the runtime requirement.
• If Tokio-only is the decision, frontend integration docs explain how the TUI should run it.
• If injection is desired, follow-up issues are created for HTTP client, clock, network listener, and storage injection.
• The public facade does not accidentally promise runtime independence it cannot support.

[artrixdotdev]

Claiming this in branch docs/document-libtortillas-runtime-boundary.